Lower Unit Oil Leak

[JimS123]

2019 Mercury 4-stroke 150 HP.

Looks like a minor leak emitting from the rear end of the lower unit, where it bolts on. The motor has very low hours and is always garage kept. Fresh water only. I noticed a slight drip of oil after sitting all Winter. The oil is puddling on top of the AV plate and dripping off the end. After 6 months of sitting I estimate the leak at less than an ounce.

My thought is to tighten the 5 bolts holding the lower unit on, check to be sure the gear oil is full, and monitor the leak.

Anybody suggest a better approach?

(The pic shows the motor tilted up - it was horizontal all Winter)

 

[Mc Tool]

1st thing to do is to be sure its gear oil and not engine oil ......no chance you spilt oil whilst topping up the engine . You can usually tell by the smell of the oil what it is .
Tightening the lower unit mounting bolts wont help .

 

[JimS123]

1st thing to do is to be sure its gear oil and not engine oil ......no chance you spilt oil whilst topping up the engine . You can usually tell by the smell of the oil what it is .
Tightening the lower unit mounting bolts wont help .

My plan was also to check the engine oil. But if it was, how did it get down there?

A spill is not feasible. I wipe everything down meticulously. I'll give it a try, but I don't think there is enough there to get a good smell.

Why won't a bolt tightening not help? I'm assuming some came loose after 6 years.

 

[jlh3rd]

those lower unit bolts have nothing to do with sealing anything. The just hold the gearcase on.
The nuts are nylocks so not prone to loosening.

 

[roscoe]

Tightening won’t help because that is not what seals the gear oil in.
There are seals on the driveshaft and the shift shaft.

“I'm assuming some came loose after 6 years.”

They should have been loosened/removed when the water pump impeller was replaced. If you haven’t done that yet, it’s time.

When you remove the gearcase to service the water pump, you can inspect to see if it is engine oil or gear lube, and whether a seal has failed.

 

[airshot]

Would be kinda unusual for gear oil to be above the cav plate....Have you checked the gear case oil for water intrusion or low level ?

 

[JimS123]

Would be kinda unusual for gear oil to be above the cav plate....Have you checked the gear case oil for water intrusion or low level ?

There was no water intrusion last Fall when I changed the oil. It has not been in the water since then.

I can see where the oil is seeping out. It's from the seam where the lower unit meets the mid section, which is just above the AC plate. I don't believe there is a gasket there. Isn't it true that the 5 bolts are what brings it tight together?

 

[Chris1956]

Jim, if it is gear oil, drop the gearcase and find the leaking seal. It could be the shift shaft seal or the driveshaft seal. If the leaking point is not visible, you could drain the gear oi and pump a few PSI of air into the gearcase and spray those areas with soapy water.

I have not had any Merc newer than 1993, but gear oil leaks have occurred over the years. My '93 Merc V6 has leaked twice at the shift rod seal. My '77 Merc IL6 never leaked there, but the driveshaft seal in the waterpump base was the leaking area a few times.

 

[Mc Tool]

That gap is where the lower unit bolts to the mid section . The lower unit is sealed to contain the oil inside itself . The engine sit atop the mid section ....also sealed to contain oil , but there is no oil in the mid section , however the engine main seal may leak down onto the mid section and out that gap . The lower unit can leak out of either the drive shaft seal or the gear change shaft seal on top of the lower unit .....and the oil will show up at the same gap .....so you have to suss weather its gear or engine oil .
That gap doesnt need to be oil or waterproof ,its just there where the sealed lower unit bolts on .